Distribution and increment of biomass in adjacent young Douglas-fir stands with different early growth rates

Total biomass increments were determined for three adjacent 22-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)plantations in the Oregon Coast Range that had widely different early growth rates. Estimated total aboveground biomass of thestands, designated slow, intermediate, and fast, was 98.7, 148.7, and 203.7 Mg • ha-', respectively; estimated mean biomassincrement in the 5 years previous to sampling was 8.9, 12.6, and 12.3 Mg •ha-1• year-1. The slow stand had a greater propor-tion of aboveground biomass in branches and a smaller proportion in stem wood than the intermediate and fast stands.Differences in biomass increment were primarily due to stem rather than crown growth. Total belowground biomass was highestin the fast stand, the difference being due to roots >5 mm in diameter; weight of roots 5 mm comprised about 77% of the total root system in those stands and 90% in the fast stand.Increment of Milts >5 ram was 2.2, 2.5, and 3.0 Mg•ha-i.year-1 in the slow, intermediate, and fast stands. The ratio ofproductivity to total leaf nitrogen suggests that nitrogen is a principal limiting resource in the intermediate stand. The fast stand,with a leaf area index 50% greater than the others, is probably limited by light. The slow stand has anaerobic soils during at leastpart of the year, which may restrict rooting depth and thereby induce water stress during summer drought.